A rocket exhaust plume consists of heated gas moving at a high speed and at a high temperature. This gas formation is inhomogeneous in structure, has a non-uniform velocity, and a non-uniform composition. Frequently, a plume contains supersonic shock waves with high gradients of pressure and temperature across the wave region. The plume characteristics, e.g., its size and shape, light emission intensity, and spectral signature, depend not only on the rocket aerodynamic characteristics and the rocket propulsion system, but also on the flight velocity and altitude of the rocket. For example, FIG. 1 shows a representation of the plume characteristics' dependence on a velocity of the rocket as shown in the prior art, and FIG. 2 shows a schematic of the plume diameter as a function of altitude as shown in the prior art.
Detectability of a rocket plume at a particular wavelength is dependent on an intensity of the emission at the wavelength, atmospheric transmittance, and the strength of the background signal. Generally, a plume can be considered as a black body radiating source with a spectral distribution characterized by the plume's temperature. The core of the plume of a supersonic tactical missile has temperatures of approximately 1500 Kelvin. However, unoxidized fuel materials typically mix with ambient air downstream of the plume core and produce a higher temperature afterburning mixing region with temperatures as high as 3000 Kelvin. At these temperatures, blackbody spectra have a non-negligible ultraviolet radiative component.
In addition to black body radiation, spectral lines due to chemical combustion of propellants can superimpose on the infrared spectra. The molecules responsible for most of the gas thermal emissions in missile exhaust plumes are water vapor (H2O), carbon dioxide (CO2), as well as formation of electronically excited hydroxyl (OH) and carbon monoxide (CO) in the chemiluminescence process:2CH+O→CO+OH*→CO+OH+hv, where OH* indicates the OH is in an excited state.
FIG. 3 shows a representative OH emission spectrum as shown in the prior art. In particular, the ultraviolet OH chemiluminescence observed during a C2H2O+O atom reaction is shown. Additionally, the table below summarizes spectra from various chemical elements.
Significant Spectral Combustion ProductEmission MechanismBand (μm)CO2Gas Thermal EmissionMid IR (3-5)H2OGas Thermal EmissionNear IR (0.75-3)COChemiluminescenceMid UV (0.2-0.3)OHChemiluminescenceMid UV (0.28-0.29)COGas Thermal EmissionMid IR (4.6-5)C (soot)Black Body EmissionMid UV (depending ontemperature), IRLight Metal OxidesGas ThermalMid UVEmission/GraybodyNa & CompoundsGas Thermal EmissionVisible (0.59, 0.68)K & CompoundsGas Thermal EmissionNear IR (0.79)
Generated plume light signatures are attenuated by ozone composition of the atmosphere, by humidity of the air, and by molecular oxygen. Additionally, sun background radiation can introduce significant noise, which for certain light wavelengths, can be comparable in amplitude with the plume's light signal. For ultraviolet radiation, there is a narrow window of radiation wavelengths between 270 to 290 nanometers that may not be attenuated by the atmosphere and/or shielded by sun radiation. For clear air, with a low ozone content, and during night time, a slightly wider range of radiation wavelengths may be available.